Add MessageEvent.data template for dart2js, remove duplicate claims on ClipboardEvent and PointerEv…

tools/dom/templates/html/impl/impl_MessageEvent.darttemplate

Index: tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
index 1b3280d287707f799102e3dfadde9311934e40b5..fec434127b637751cd4056633f02483ec980c4fe 100644
--- a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
@@ -36,6 +36,22 @@ $if DARTIUM
   @DocsEditable()
   dynamic get data => convertNativeToDart_SerializedScriptValue(
       _blink.BlinkMessageEvent.instance.data_Getter_(unwrap_jso(this)));
+
+$else
+  // TODO(alanknight): This really should be generated by the
+  // _OutputConversion in the systemnative.py script, but that doesn't
+  // use those conversions right now, so do this as a one-off.
+  @DomName('MessageEvent.data')
+  @DocsEditable()
+  dynamic get data => convertNativeToDart_SerializedScriptValue(this._get_data);
+
+  @JSName('data')
+  @DomName('MessageEvent.data')
+  @DocsEditable()
+  @Creates('Null')
+  @Returns('Object|Null')
+  final dynamic _get_data;
+
 $endif
 
 $!MEMBERS